Diaphragmatic morphological post-mortem findings in critically ill COVID-19 patients: an observational study.

Our study investigates the post-mortem findings of the diaphragm's muscular structural changes in mechanically ventilated COVID-19 patients. Diaphragm samples of the right side from 42 critically ill COVID-19 patients were analyzed and correlated with the type and length of mechanical ventilation (MV), ventilatory parameters, prone positioning, and use of sedative drugs. The mean number of fibers was 550±626. The cross-sectional area was 4120±3280 μm2, while the muscular fraction was 0.607±0.126. The overall population was clustered into two distinct populations (clusters 1 and 2). Cluster 1 showed a lower percentage of slow myosin fiber and higher fast fiber content than cluster 2 (68% vs. 82%, p<0.00001, and 29.8% vs. 18.8%, p=0.00045, respectively). The median duration of MV was 180 (41-346) hours. In cluster 1, a relationship between assisted ventilation and fast myosin fiber percentage (R2=-0.355, p=0.014) was found. In cluster 2, fast fiber content increased with increasing length of the controlled MV (R2=0.446, p=0.006). A high grade of fibrosis was reported. Cluster 1 was characterized by fibers' atrophy and cluster 2 by hypertrophy, supposing different effects of ventilation on the diaphragm but without excluding a possible direct viral effect on diaphragmatic fibers.